Wireless communication networks are utilized for a variety of different applications, including the transfer of voice and data information between the users of such networks. For example, advanced wireless networks are utilized for wireless communication applications, such as cellular systems, including cellular telephone systems, and other wireless networks, such as the more recent WiMAX networks.
In general, a wireless communication network utilizes a fixed network that may include or be functionally coupled with existing communications networks, such as the established Public Switched Telephone Network (PSTN) or other networks having a fixed functionality for backbone transmission. The fixed network is coupled through a switching network to a plurality of individual and remote base-stations. For example, a base-station may define a cell in a cellular system. The customers' devices, such as mobile phones, and other devices, interface with the base-stations for communication purposes. Generally, existing base stations are low-power, multiple-channel two-way radios that are in a fixed location. For example, when a call is made on a mobile phone device, the call is routed to a nearby base-station. From that base-station, the phone call is connected into a network, such as a regular land-line phone system or into a mobile phone network, via the switching network. To date, base-stations in a mobile phone network or cellular network are often referred to as a cellular phone tower, generally referring to the tower or support on which the antennas of the base-station are mounted. Each base-station generally comprises a number of transmitters and receivers, or transceivers.
FIG. 1 illustrates a block diagram of a basic base-station architecture. Generally, the base-station 10 includes a base 20 and a tower or support structure 22. The base may comprise the various channel electronics and transceiver electronics for the base-station which are located in a housing or shed positioned proximate the base of the tower or support structure 22. In FIG. 1, the tower or support structure is indicated as a tower top, as oftentimes the antennas for a base-station are positioned at or near the top of the tower or support structure. However, while it is generally desirable to position the antennas at the highest possible point for better reception, they might be positioned anywhere on the tower or support structure. Coupling the base electronics 20 with the tower top or tower 22 are coaxial cables 24.
Signals to and from a fixed network, that are interfaced to the base-station through a switching network, are indicated by reference numeral 26. On the transmitter side of the base station, often referred to as the down link, voice signals and/or data signals are encoded by appropriate channel cards. The channel cards include appropriate encoder circuitry 30 that encodes the signals according to the chosen wireless standard, such as GSM, CDMA, WCDMA, etc. The encoded signals are then digitally modulated by appropriate modulator/demodulator electronics 32 and then are digitally upconverted to a composite analog signal by appropriate conversion circuitry 34 (DUC). The composite analog signal is then directed to the appropriate transceiver circuitry 36 of the base-station electronics 20. In transmission, the composite analog signal is generally amplified by an appropriate high power amplifier or amplifiers to a sufficiently higher power level for transmission through the antennas on the tower 22. Because of the distortion introduced into the signal by such high power amplifiers, a linearization scheme will generally be utilized. For example, digital predistortion (DPD) might be used to linearize the signals coming from the high power amplifiers. Generally, such digital predistortion anticipates the distortion that is introduced by the high power amplifiers and predistorts the input signal to the amplifiers so that the amplified signal is more linear and the overall distortion is reduced. Generally, the digital predistortion is performed after the signals have been digitally modulated and upconverted, but before they are amplified. The signals are then directed on one or more transmit coaxial cables 38 to the tower or tower top 22 where they are transmitted via one or more antennas 40. Also at the tower top, appropriate filter/duplexer circuitry 42 might be utilized, such as to separate the transmit and receive signals. The transmitted signals are then received by appropriate customer equipment, such as mobile telephones.
On the receiver side or the uplink, signals are received from the mobile devices and are captured by one or more receive antennas 46. The uplink signals are received, filtered, and directed to the appropriate receiver circuitry through filter/duplexer circuitry 42. Signals are routed by the appropriate coaxial cables 24 to transceiver circuitry 36. Generally, the transceiver circuitry 36 includes one or more low-noise amplifiers (LNA) that are used to amplify the received signals. The signals are then mixed down to an intermediate frequency (IF), digitized, and demodulated using the digital demodulator circuitry 32. The demodulated channels from channel cards 28 are then routed to appropriate circuitry via a switching network and lines 26. While the transceiver block assumes a single block and combines elements of a transmitter and a receiver, the circuitry might also be separate circuits for such transmit and receive functionality.
Conventionally, all the active electronic components of the base-station, except for the transmit and receive antennas 40, 46 and some filtering/duplexing circuitry 42 are located at the base of the tower 20, such as in a housing or shed. Coaxial cables 24 couple the base-station electronics 20 with the top of the tower or other support 22 and the appropriate antennas 40, 46 as noted. For the transmission function, the coaxial cables must be capable of carrying high power RF signals to the tower top 22 for transmission using directional and/or sectorized antennas 40. However, as is well known in the art, a large amount of the transmit power is lost in the coaxial cables 24 because of cable impedance and transmission loss.
It has been proposed to position amplifiers at the tower top 22 and very near the transmit antenna elements 40 to reduce the transmission loss and thereby increase the power efficiency and reliability of a base-station. However, amplifiers positioned at the top of a tower along with the antennas to form what is often referred to as an “active antenna” arrangement are not as wide-spread as desired due to various technical considerations that must be addressed. First, the tower-top electronics must be highly reliable to avoid frequent trips by maintenance personnel. Furthermore, linearization is often required to linearize the output of the power amplifiers due to the requirements of various modulation and transmission schemes. For example, digital predistortion might be utilized with a high power transmit amplifier. As such, linearization electronics, such as digital predistortion might have to be implemented along with the amplifier at the tower top. This requires additional electronics at the top of the tower along with the high power amplifiers, and all such electronics would have to be properly cooled, such as convectionally cooled. Currently, the amplifiers and existing base-stations do not meet these requirements, and, hence, the amplifiers still remain at the base of the tower and the resulting loss of efficiency in the coaxial cables is tolerated.
With the receiver circuitry, the coaxial cables are used to carry the signals from the receive antennas 46 at the top of the tower to the base 20, where low noise amplifiers (LNAs) and the digital demodulators are located. In some architectures, the LNAs might be placed at the top of the tower next to the receive antennas 46 in order to improve the sensitivity of the receiver. Unlike power amplifiers on the transmitter side, the LNAs produce significantly lower power signals. Nevertheless, the LNAs are also required to be highly reliable if they are placed at the tower top 22.
Therefore, there is a need to address the power losses associated with base-stations utilizing conventional base-located electronics, tower-top antennas and coaxial cables. There is further need to address such power efficiencies, while still maintaining the linearity of the signal transmission.
Another aspect to be addressed involves the frequency allocation standards that are utilized for the wireless communication system. In some wireless standards, separate frequencies are assigned for the transmitter and the receiver, thus enabling them both to function simultaneously. Such a standard is termed a Frequency Division Duplex (FDD) system. Currently popular standards, such as CDMA, GSM, IS, IS-136, and WiMAX, are examples of such FDD systems.
Alternatively, the transmitter and receiver signals might be positioned in the same frequency band, and they are multiplexed in the time domain so that only one of them is operational. That is, at any given time, the base-station is either receiving or transmitting signals. These systems are termed Time Division Duplex (TDD) systems. As such, improvements to base-station efficiency must also address these various wireless standards that are utilized for any particular application.
The present invention addresses the needs in the art and provides an overall improvement in the efficiency of a base-station, while maintaining the desired linearity of the high power transmission signals.